Medical imaging is important for enabling early diagnosing of many diseases and X-ray detectors are widely used for this end. X-radiation is absorbed at different rates in different tissue types such as bone, muscle and fat, forming an image that can be examined by a physician in diagnosing purposes. The importance of obtaining as accurate images as possible is readily understood. Further, X-radiation may be harmful in larger doses and it is therefore important to minimize the X-ray dose that a patient is exposed to during an examination.
In view of accuracy of the images, a collimator or diaphragm or aperture constitutes an important part of an x-ray apparatus. A collimator is a device including a material that significantly absorbs X-radiation and that serves to gate or collimate beams as well as to shield from scattered radiation. It is designed to filter a stream of rays so that only those entering the openings of the collimator in a certain direction are allowed through and all other rays are absorbed. Without a collimator rays from all directions would illuminate the patient giving unnecessary high radiation dose. Using a collimator thus ensures that only useful X-rays are irradiating the patient, hence reducing the radiation dose. Furthermore, the collimator can be used to produce narrow sheets or beams of X-rays improving the position resolution of some type of X-ray detectors where the width of the incoming X-ray beam defines the position resolution rather than the pixel size of the X-ray detector.
Typically, a collimator is a thick sheet of some radiation-absorbing material, such as lead, with one or several thin slits machined or etched through it. There are several considerations to pay attention to when making a collimator in order to obtain a high quality image and minimize the radiation dose that the patient is subjected to. In order to absorb X-rays efficiently, the sheet from which the collimator is made cannot be too thin, although it would be favourable in view of consumption of material and related costs, and also since a lighter collimator would be easier to handle. A difficulty when making a collimator is undercut, i.e. the lateral etching that occurs as the etching proceeds vertically. The thicker the material the more pronounced is the undercut problem, i.e. it is difficult to increase the thickness of the sheet and maintain a small and uniform slit. The ratio of the thickness of the sheet to the width of a slit is known as the aspect ratio. However, a thinner sheet entails other difficulties in the production of the collimator, since a thin material is more prone to warping and obtaining altered dimensions than a thicker one, which affect the precision of the collimator. Further, a too thin collimator is not feasible since undesired radiation would penetrate the collimator resulting in a deteriorated image quality and also in the patient being subjected to a higher radiation dose.
A collimator should pass substantially parallel radiation originating unscattered from the X-ray source and absorb non-parallel radiation that e.g. has scattered between the X-ray source and the collimator. To meet the second requirement, the sheet should be of adequate thickness for absorbing the non-parallel radiation.
Further, the manufacturing of a collimator is a work requiring high accuracy and precision, comprising forming slits of dimensions down to a μm range, and it is difficult to obtain an adequate accuracy. Such precision work is additionally very costly and requires expensive tooling, which adds considerably to the cost of an X-ray apparatus.
A collimator can be manufactured in a vertical or horizontal lamellar structure, i.e. a number or thin layers are prepared individually, each having the desired pattern. Thereby the difficulties related to undercut is avoided. However, the precision may still be inadequate since it is very difficult to stack the different layers on top of each other with maintained precision.
All the above-mentioned factors and difficulties related to the manufacturing of collimators ultimately affect the performance of the X-ray apparatus and an improved method of making a collimator would therefore be desirable.